1. Field of the Invention
This invention relates generally to the field active filters. More particularly, this invention relates to the field of active filters suitable for use as phase shifters in quadrature demodulators.
2. Background
As progress is made toward miniaturization of FM transmitters and receivers, the FM demodulator remains one of the most difficult circuits to miniaturize. This is primarily due to the moderately high frequencies that are normally involved and the inability to reduce the size of inductors of suitably high inductance and Q factor for use in such demodulators. As the size of such devices diminishes it is also important that FM demodulators be capable of operation at very low voltage and current levels since battery size is a basic limitation in miniaturization of such devices.
Some types of FM slope detectors and the like also utilize crystals or ceramic resonators as portions of a demodulation circuit. This is as disadvantageous as using an inductor due to the fragility, size and expense of such devices.
The inductors used in quadrature demodulators are among the costliest, heaviest and least reliable components used in miniature receivers such as paging receivers and the like. It is therefore very desirable to eliminate their use in small and even in large electronic devices.
There are some demodulators available which may be implemented without the use of inductors, such as the phase locked loop and pulse counter type demodulator. Unfortunately, these demodulators suffer from a number of drawbacks including the inability to operate at the very low voltage and current levels necessary for battery operated receivers such as paging receivers. They are also frequently low Q devices exhibiting inferior noise performance. The pulse count demodulator has the further disadvantage that, except for operating frequencies below approximately 200 kHz, it provides a very low amplitude recovered signal. It is therefore desirable to provide a quadrature demodulator which may be fully implemented in integrated circuit form and operated at low voltage and current levels. The quadrature demodulator is frequently used in FM communications applications due to such desirable characteristics as high audio output and high signal to noise ratio. It is therefore very desirable to provide an inductorless version of a quadrature demodulator which may be fully integrated on a single integrated circuit.
Unfortunately, integrating a quadrature demodulator pfesents a number of technical challenges which must be overcome. This is especially true when the demodulator must be operated reliably over a wide range of temperatures and when steps must be taken to insure trimmability of the demodulator to assure optimum performance under all environmental conditions and integrated circuit processing variations. Under these conditions, highly temperature stable circuitry is required and the ability to precisely control temperature coefficients independent of the individual component temperature coefficients is very important. It is also important to have the ability to adjust circuit performance to account for variations in integrated circuit processing parameters, that is, to initially tune or adjust the circuit to overcome manufacturing variations in component values. The present invention provides solutions to these problems and others.